“Recent discoveries,” says Humboldt, “have shown that the ocean has its currents exactly as the air. Living, as we do, upon the surface, they have been beyond our reach; but now, having obtained soundings to the depth of four miles, we have ascertained that there is a rush of icy water from the Pole to the Equator, just as there is a draft of air close to the earth into the centre of Africa. The Mediterranean offers an apparent anomaly of a higher temperature at great depths. This Arago explains by the fact, ‘That the surface of the water flows in as a Westerly current, whilst a counter current prevails beneath, and prevents the influx from the ocean of the cold current from the Pole.’[27] If there was nothing to determine the currents at the entrance of the Mediterranean, save the relative degrees of cold at great depths between it and the ocean, the cold water would run in at the lowest depths, and the warm water would run out on the surface, which is precisely the reverse of what it does. Here is a body of water 740,000 square miles in extent, subject hourly to the increase of its specific gravity. Upon the surface, a crust of salt is left in the course of every year, sufficient to give a double charge to the depth of six fathoms. To adjust the difference thus created with the ocean, there is but a narrow inlet,--a mere crack upon the side of the vessel, an interval of six miles left in a circumference of four thousand. By this, in its deepest part, the heavier water will have to find its way out, and thus occasion an indraught of water above, besides the demand created by evaporation. It remains to be ascertained by experiment, that the specific gravity of the water in the Straits varies at different levels, and at what level it commences to move outwards. These experiments will present great practical difficulties from the tides at the sides, which will mingle the streams; and, from the shallowness towards the ocean, they must be made in the middle and at the Mediterranean side. The evaporation, and the differences of specific gravity, will give the means of calculating the amount of water passing through in both directions, and the depth and velocity of the two currents. But it may be inferred that the currents will have the greatest speed at the top and the bottom,--that their velocity will diminish towards the centre, and that a neutral space of dead water will remain, not only in consequence of the counter-impetus of the currents, but because of the nearer approach of specific gravity, and the mingling of the two waters, which would destroy the moving power. With this solution we can at once understand the powerlessness of tides and storms, currents without difference of level, or prevalence of winds: the volume of the stream is accounted for, the mass of salt disposed of, and the apparent rebellion against the laws of Nature put down.